I would love to have had an extension like this back in the 90's .. so I didn't have to wrap my cart extension cable in foil, and keep it rather short in order for it to work well. Long story short: 130XE, MIO, XF551, Gumby, Transkey 1 in an AT tower case. Cart port was routed to the front 3.5" slot.
I'd buy a full kit now too, for another project that is sitting in the to-do pile.
Sounds like you had the same issues as I did with my early put an 'A8 in a PC box', and how to get the Cart to work reliably. This is where this project really makes a lot of sense having the active buffers. So yes I would definitely vote for doing this for the Cart, but not so much for the ECI. This way you can keep it compact, and relatively inexpensive at the same time. Also I suspect it'll be more universal, better able to fit in any system.